Hi there, welcome to BYC!
I would say not less than 21 days before you start collecting from the new stag. I easily get fertility for 10-14 days after separation. There is some beyond that, but not enough or not important enough for me to try hatching beyond that. But enough that if I didn't have extra energy or space for possible cross breeds.. I would wait at least 21 days. 30 days *should* guarantee your genetics.
Another way to determine would be separate the hen and start cracking her eggs open after a couple weeks and look for the blastodisk or blastoderm to see if eggs are fertile or infertile before adding your new rooster. And then start collecting a couple days after adding him.
At my place... I keep all my stags in one pen and all ladies in another. So all hen eggs are unfertilized and all the boys get along. I add the stag I wish to hatch from to all the ladies, just for the day and return him to his stag flock (after roost while dark) before pecking order gets out of whack. I then collect eggs from only the hens I wish to hatch from... I have kept about 5 different breeds in only 2 yards... keeping different colored layers helps with this tactic. But with this set up, if I want to do individual test matings.. no waiting.
Happy chicken-eering!